Geophysical exploration



Sept 1952 A. A. BRANT ETAL 2,611,004

GEOPHYSICAL EXPLORATION Filed Sept. 15, 1948 6 Sheets-Sheet l g. l 3 2072111741. POTENTIAL DISTRIBUTION NE 0/! T R/BUT ION TU APPLIED DUE TO GENERAL APPL IEO GENERAL I PgLEQLRAZA T/ON POLARIZATION s L 3 3 k s E k fi/iTAA/CE ELECTROD Q o 21 TOWARD 22 1.1 2 FIELD DUE TO DOUBLE LIYER CHARGE EFFECTS POSITION POSITION 0F 47' PART/OLE SURFAOES TABLE OF OVERVOLTAGE VALUES APPARE/VT DENSITY- MPS PM sac 0.001 0.01 0.1 1.0

ME TAL OVERVOLTAGE 111 v01 79 040111011 0.90 1. 1. 22 1.25 11121700177 0. 90 1. 04 1. 07 1. 12 711v 0. as 1. 0a 1. 22 1. 25 015410 711- 0. 1.05 1.14 1. 25 z11v0 0.72 0.75 1.00 1.25 LEAD 0.52 1.09 1.10 1.20 ALUMINUM 0.55 0. 1.00 1.29 01741 11175 0.50 0. 7a 0.90 1.22 SILVER 0.47 0.70 0.09 1.09 001 252 0.48 0. 53 0.00 1.25 IRON 0. 40 0.55 0.05 1.29 00w 0.24 .059 0.59 0.90

ARTHUR A. BRANT EVERETT A. mas/717 INVENTORS A T TOR/V5 Y Sept. 16, 1952 A. A. BRANT EIAL 2,611,004

I GEOPHYSICAL EXPLORATION Filed Sept. 15, 1948 6 Sheets-Sheet 2 l/ l I P SURFACE OF GROUND ARTHUR A. BRANT EVERETTA. GILBERT IN VEN TORS ATTORNEY p 1 A. A. BRANT ETAL 2,611,004

GEOPHYSICAL EXPLORATION Filed Sept. 15, 1948 6 Sheets-Sheet 4 POM-R SUPPLY -o r o c W77: o AND CONTROL REGORDER PUL SER- Y MULT/V/BRATOR .13. ff CLIPPER, YYII 1'" 0 P3 0/ SURFACE ""1" *4/17 or enouwo 15 70 c/MRa/Na I 2 ascmooss r0 PuLsn- 37 HMTIWIRATOR ARTHUR A. BRA/VT EVERETT A. GILBERT IN V EN TORS BYW%W ATTORNEY p 16,1'952 A. A. BRANT ETAL 2,611,004

GEOPHYSICAL EXPLORATION Filed Sept. 15, 1948 6 Sheets-Sheet 5 T0 OPERATING COIL OF HIGH VOLTAGE RELAY A AAA 300 v l'i'l'l'l'] ma /(i; 200K 200K :5 v,

gal T0 CORRESPONDING TERMINALS 0N AMPLIFIER cmoulr FIGURE I5 ARTHUR A. BRANT EVERETT A. GILBERT INVENTORS A 7' TOR/YE Y Sept. 16, 1952 A. A. BRANT m-AL 2,611,004

GEOPHYSICAL EXPLORATION I Filed Sept. 15, 1948 6 Sheets-Sheet 6 A TTOR/VE Y Patented Sept. 16, 1952 GEOPHYSICAL EXPLORATION Arthur A. Brant, Toronto, Ontario, Canada, and

Everett A. Gilbert, Lake Hiawatha, N. J., assignors, by mesne assignments, to Geophysical Exploration Company, a corporation of Delaware Application September 15, 1948, Serial No. 49,354

13 Claims. 1

This invention relates to geophysical exploration and more particularly to a novel method and apparatus for the detection and location of scattered or concentrated metallic, mineral and carbon particles embedded in a medium permeated by an electrolyte.

Although the art of geophysical exploration includes numerous electrical methods for the determination of sub-surface conditions we shall refer briefly to two which, in a broad sense, will provide a background for the proper understanding of the present invention. I

United States Patent No. 1,163,468 issued lDecember 7, 1915, to C. Schlumberger, may be taken as representative of the early methods proposed for the detection and location of minerals below the ground surface. The method consists in maintaining a difference of potential between two charging electrodes on the surface of the ground and measuring the resultant potential difference appearing between two pickup electrodes spaced apart but disposed Within the limits of the charging electrodes. The potential difference appearing across the pick-up electrodes will vary with the magnitude of the current flowing through the earth between the charging electrodes and with the nature or character of the sub-surface conditions. Such potential variations provide the data from which certain deductions may be made with regard to the geological nature of the sub-soil. Inasmuch as the potential readings of the pick-up electrodes are taken during the period when the chargingcurrent is flowing through the ground this method fails to distinguish between the effects of many related and contributing factors with consequent reduction of diagnostic value. Later workers in this field proposed various refinements based essentially upon the detection of the charging and discharging potentials effective as a result of electrical current flow through an electrolyte medium. These methods fall into a category referred to as electrolytic polarization methods and may be represented, for purposes of this discussion,by United States Patent No. 2,190,321 issued February 13, 1940, to G. Potapenko.

The Potapenko method comprises the insertion of two charging electrodes at spaced points in the ground. Two or more pick-up electrodes are also inserted into the ground but at points lying between the charging electrodes. A. D. C. charging current is fed into the ground through the charging electrodes and this current results in an ionic polarization of the sub-surface electrolyte medium lying within the range ofthe charging current flow. The flow of charging current is then interrupted after which any potential appearing across the pick-up electrodes is detected by suitable means, said potential being due to the discharging of the E. M. F. of polarization established in the electrolyte during the flow of the charging current. The character of the discharging E. M. F. of polarization is related to the nature of the electrolyte and data thus obtained is employed to determine geological sub-soil conditions.

Electrical methods of exploration may be considered as indirect in principle as they involve a critical interpretation of the data obtained in' order to arrive at a conclusion with respect to the type, nature and character of the sub-surface conditions, strata, deposits, etc. The accuracy of interpretation of electrical prospecting results depends upon the ease with which interfering factors may be eliminated. While the electrolytic polarization method is adapted for oil exploration, due to the polarization effects arising from the porosity differences (per cent electrolyte) of the formations concerned, it is not satisfactory for the detection and identification of scattered mineral, metallic and car-' bon particles as contemplated by this invention. These substances are generally found in nature as sulphides and we have found that a practical method and apparatus for the detection of such substances must inherently involve means for ascertaining what may be termed effects of the second order of refinement. A more thorough understanding of our invention will be had from the following detailed description. For the present it will suffice to point out that our method and apparatus for geophysical exploration is based upon an electro-chemical effect identified as the Overvoltage Effect or electrical double layer charging efiect occurring at the surfaces of metal particles in an electrolyte.

An object of this invention is the provision of a method and related apparatus for geophysical exploration whereby the presence and specific location of scattered or concentrated metallic, mineral and carbon substances lying below the ground surface may be detected and identitied with a degree of accuracy hitherto impossible.

An object of this invention is the provision of an electrical method and apparatus for geophysical prospecting employing the overvoltage effect or double layer charge effect at the surfaces of metallic, mineral and carbon particles suspended or embedded in a liquid medium.

An object of this invention is the provision 3 of an electrical method of geophysical exploration which method comprises impressing a D. C. charging current through the ground between spaced charging electrodes, interrupting the charging current aftera predetermined time interval, and. measuring the resultant discharge potential due solely to the double layer charge effect at solid particles suspended in a liquid medium.

An object of this invention is to provide a method of measuring the overvoltage or double layer charge effect at solid particles suspended or embedded in a liquid medium as a result of a charging field impressed through the electrolyte.

An object of this invention is the provision.

of a method and apparatus for indicating the presence of free metals and conducting metallic compounds in a medium, consisting entirely or inpartof a conducting solution of waterand soluble metallic salts.

, An object of this invention is the provision, of.

apparatus for geophysical prospecting and com:

prising means for impressing a flow of D. C. cur-- rent into, the ground, means for detecting the.

overvoltage or double layer potential at subsurface particles and timin means. eifective. to

cause operation of the detecting meansa. predetermined time interval after the flow of charg-' ing current has been cut oif.

.An object of this invention is the provision of apparatus for geophysical exploration said apparatus .comprising means for impressing. a pulse of D. 0. current through a giv'envolume of ground, means for measuring and recording the. resultant potential due to the double layer.

chargeeffect manifest at the surface of metallic particles embedded in a. liquid medium. and

means effective to eliminate unwanted transients.

resulting from effects otherthan the double layer charge phenomenon whereby data thus obtained face thereby establishing an electric field through.

the ground, an electronic amplifier having its input terminals connected to spaced pick-up electrodes disposed in the ground and between. the. charging electrodes, means efiective periodi.-

cally to remove the electric field, means eflective to reduce the amplifier'gain to zero during the period when the electric field is established, means effective to Open the amplifier gain a predetermined time after the establishment of the electric field, and means for indicatingthe output of saidamplifier.

These and other objects and advantages will become apparent from the following description when taken with the accompanying drawings which illustrate the principles of operation and apparatus suitable for practicing the invention. It will be understood thespecific detailed description of the illustrative apparatus and method is not intended to limit the invention beyond the terms of the claims appended hereto.

In. the drawings wherein like characters do.- note like parts in the several views:

Figure 1 illustrates the electrolyte polarization phenomenon; Figure 2.; shows the relative nature-pf the ap- 4 plied and general polarization fields of the Fig. 1 arrangement after a given time interval;

Figure 3 illustrates the overvoltage or double layer charging phenomenon at particle surfaces;

Figure. 4..shows the relative. .natureof the applied, general polarization. and double'layer fields of the Fig. 3 arrangement; Figure 5 is a table showing maximum overvoltage values for certain metals when the electrolyte is 2N.H2SO4 at 25 C. and at various current densities;

Figure fi-illustrates factors employed in developing the formula with respect to the potential of a point outside the surface of a metallic particle. and due to the double-layer charge;

- Figure *7 illustrates" the factors employed in developing; the formula for the electric field components as related to a spherical particle in a field of uniform current flow;

Figure 8. illustrates, type. curves for developing. the formula for the. overvoltage potential '..at a point P L Figures illustrates a simplified arran ement to explain the fundamental developmentv of apparatus adapted for useinpracticing our method of geophysical exploration; Figure 10 illustrates aseries. of curves appli-- cable to the arrangement of Figure 9; K

Figure, 11 is a. plot of the overvoltageeifect with respect to equal displacements of theelectrodesystem along theg-round and, showing how our method determines the center ofa. sub surface. mass;

Figure 12 is a block diagram illustrating, in a broad sense, the, adaptationhf our apparatus-to.

the measurement of double layer charge effects electrolyte gives rise to polarization defined as the migration of the electrolyte charges in such fashion thatthe positive ionsmove towardthe negative.(- electrodeand the minus ions; move toward the positive electrode. Thereby electrolyte concentrations are set up throughout the liquid and charge concentrations at the current electrodes. This condition is illustrated in Figure 1 wherein the current, or charg-v ing, electrodes-2i and 22 are immersed in theelectrolyteand are connected to, opposite sides of a;

battery 23.. Since charges are transported, differ.

ences in concentration result which in turn gives rise to a potential difference. This results in afieldbeing superimposedthroughout the liquid. medium in addition to the normal initially applied field. It is to. be noted this general polarization field. is directed in a sense opposite to. the applied. D. C. field.

I When the-normal'field is removed, in a matter of microseconds, by opening of the switch. 25,-- Figure. 1, the electrolytic polarization will.dis-- charge in a matter of seconds in adirection-opwirin diagram of. the -pulsar posite'to the initialcharging field. The'dashed curve of Figure 2 illustrates the relative potential distribution of the general polarization field along the line between electrodes 2| and 22. Thus, a pair of pick-up electrodes 25, 26, Figured,

in contact with the liquid will have a polarity reverse to the polarity of the charging electrodes 20, 2|, as shown.

The discharging potential of polarization may be indicated or measured by suitable apparatus such as for example,the voltmeter 21 connected across the pick-up electrodes 25, 26. Various refinements in the apparatus employed to measure the polarization discharging potential have been proposed with the result that the slope of the discharging potential distribution can be determined. By extensiveexperimentation the character of the discharge potential is related to known conditions such that data obtained in the field may be employed as a guide to the determination of geological sub-soil conditions. However, as has been stated hereinabove, the degree of success which may be expected of electrical prospecting methods depends upon the ease with which interfering factors may be eliminated from the final, measured values. In this respect the electrolyte polarization method of geophysical exploration fails as the results obtained give no special significance to important factors which we have found are directly related to general, sub-soil conditions, and as will now be explained.

Overvoltaige or double layer charge ffects In addition to the general electrolytic polarization effect, above described, an effect known as the overvoltage, double layer charging or induced polarization effect may take place at discrete metallic particle surfaces when such particles are embedded in a medium containing an electrolyte and are subjected to an electrical field. In general, it may be added, these particles are better conductors than the surrounding medium.

As shown in Figure 3, a D. C. charging field will induce, in a good conducting metallic particle, charged surfaces with the positive surface nearer the minus current electrode. To this positive surface of the particle nega-- tive ionic charges will be drawn. For all practical purposes any two metallic particles and an electrolyte between them will form a simple electrolytic cell. If the charging current is now removed the overvoltage effect or. double layer.

charge effects at particle surfaces will discharge in a matter of tenths of a second, the discharge being represented by the neutralizing fiow of negative ions from the negative region of the solution near one side of the metallic mass tothe positive region of the other side of the metallic mass. The discharge of the overvoltage effect may be likened to the discharge of a leaky condenser. Thus, during the dischargeportion of the cycle the polarity of the pick-up electrodes 25, 26 will be as shown in Figure 3, from which it will be noted that the discharge potential due to this overvoltage or double layer effect is in the same direction as the initial charging field, as

distinguished from the discharge potential due to general electrolytic polarization, the latter being opposite to the initial charging field Figure 4 illustrates the relative voltage distribution along the line between theelectrodes Zland 22 due to theapplied normal field, the general polarization field and the overvoltage effect. The method to be described is designed to balance out the general electrolytic polarization-and measure only- 6. the'doubl'e layer charge effects at the particle surfaces.

v Metallic and mineral substances are often found in the ground in the form of sulphides. These sulphides, as Well as carbon, form relatively good electrical conductors. It is known that most rock formations below the water table are fairly well saturated with moisture. Even the rock and soil formations above the water table may contain a certain amount of moisture residue adhering to the deliquescent clay particles of rock and soil. Thus, there may be present under the ground surface a medium consisting in part of a conducting solution of water and soluble metallic salts. We employ the overvoltage or double layer' charge effect to detect and locate free metals and conducting metallic compounds. The apparatus employed for charging selected areas of theearth and for obtaining overvoltage indications is such that the data obtained therefrom is possible of interpretation to determine the specific mineral or metal lying hidden below the ground surface and the presence of which could not be ascertained by prior methods and appa ratus employed in this field.

There now follows a more detailed discussion of the double layer charging effect. When a metallic particle is placed in an electrolyte contain ing metallic ions a potential difference across the particle-solution interface immediately develops measured by the work done to remove a metal ion into the solution. Actually four steps can be considered as involved in the process. If the solution pressure of the metal is great, positive ions will tend to go into solution and build up a positive layer around the metallic particle leaving a negative layer on the metal. If the osmotic pressure of the metallic ions in the solution is larger, positive ions will deposit on the metal making a positive layer here and leaving a negative layer in the adjacent solution. In either case an elec-. trical double layer is formed. This explanation, in general terms can be expanded by classical physical or quantum mechanic considerations.

Now, if a current starts to flow ions migrate I through the electrolyte but practically no ions succeed in crossing the double layer. The result of the flow of current is thus to increase the charge on each side of the double layer and, therefore, to increase the potential difference across the metallic particle-solution boundary. An abnormally charged double layer results; that is, there is an increased potential drop (overvoltage) across the metallic particle-solution interface.

The double layer charging or overvoltage will increase either until the applied field at the metallic particle-solution interface balances the applied external field, or until the overvoltage potential is reached at which electrolytic decomposition occurs. Since in most field applications the applied field (voltage gradient) is small, the former condition will likely result.

The metals may be grouped in the order of their overvoltage eifects. Such a table showing a decreasing overvoltage of metals in a solution 1. The overvoltage varies directly with the current density.

- per unit area.

Mathematical formulation We shallnow proceed to a mathematical formulation of the overvoltage effect. The. true current density'will vary over the surface .of the metal particle and as we are interested in the. region of smallcurrent densities we apply a correction factor as indicated below. The treatment to. follow will consider the electromagnetic field associated with. the charging current as being set up and decaying instantaneously. This is justified since. the time of decay of the field isinQthe order of microseconds in the usual case whereasthe decay of therovervoltage is found to be in the order of tenths of a second.

We assume that when the electromagnetic field is established current flows in accordance with Ohms law in the solution and through the metallic particles inthe solution.

As previously stated, it, is found that the overvoltage at a point on .a metallic surface increases linearly with time when a steady D. C. current is, first caused toflow through the interface. This is'interpreted as meaning that, the first part of the current flow goes entirely toward establishment of the double layer charge at the interface between themetal and the solution. Often it is found that the apparent area of a surface is somewhat less than the true area because of minute crevices, etc., in the surface. Suppose we denote by-a the-ratio of apparent to true surface area of the metallic particle. Then a. is always less than unity (1). If i equals the apparent surface density-of current then in equals the realsurface density. Now assume the field has been establishedfor a time t; then the total surface charge per-unit area (of effective surface) brought up to the surface (positive charge from one side of the double layer and a negative charge from the other) is p=uit; If Zbe the separation of the charges on the double layer then l =r=ailt where 'r equals the moment of the double, layer double layer will be:

This overvoltage "does not increase indefinitelywith time however, for each value of. ai there is a limiting value of V obtainable. The relation The potential difference across the.

betweenapparentcurrent density and; overvolte age (measured in volts) isexpressedas:

I i=-"K 6' where the constant bisfound to be approximately F (IV) inwhich F=Faradayfs number,

R=universal gas. constant, and T=absolute temperature of the solution.

This relation holds fairlywell for rather large values of i. For small current values Equation III; above; gives largevalues of V instead of cor.- responding-small values and hence for outputposesthe equation breaks down; If, however,- we write Equation III as:

and Equation V. ininvertedform as:

-- --10g [1-+- (VIII) The time-required for the :overvoltage to build up to its'maximum value is given by equating Equations. II and VIII for V and we get:

RT i tg-m log where'tc-equals'the time of build up of the overvoltage- Now suppose the applied E. M. F. is removed. We assume now that the only current flowing acrossthe double layer is one tending to discharge it and formed of the charge of the original layer. As before we have:

and diiferentiating this with respect to time-we obtain 1 Now let us assume: that on the decay the relationship expressed in Equation IX holds. This is. strictly without justification theoretically but for larger values ofit the results based on this supposition are found to be borne out experi 1mentally. Substitutingfori from Equation V we ave:

g cirrr -r) X1 rating, this we obtain:

[ u=i1a 1ew m whereVo is the overvoltage value when the ap plied was removedi. e, at :t-=0.

9 For small bVo and hence correspondingly smaller .bV. we have, to a first approximation;

v=voe =voei (XIII) where p=time constant of decay of the overvoltage.

For K small [1 is large and also the overvoltage is large; hence a larger overvoltage metal should have a slower rate of decay than one with a smaller overvoltage (for identical ml). A very rough metal surface (a small) will have a larger time constant than a smooth sample of the same metal, and also a lower overvoltage because of the difference between apparent and true current densities.

Domains of application of formulae When a known current is passed for a certain length of time through the interfaces between the electrolyte and the metal particles the overvoltage value at the end of this interval will depend on whether or not the overvoltage has had time to build up to its maximum value for that particular currentdensity. If it has not then we-say we are working in the time unsaturated region of the overvoltage-current-time surface 6 (XIV) and the relation between overvoltage and time t n can be expressed as 1 I V=-Est (XV) where E is the electrical field normal to the interface;

s is some positive constant= and t is the time the field has been established.

The transformation from Equation II to XVI is accomplished by noting that Z' o'E where o' is the conductivity of the electrolyte rock medium.

If, however, the overvoltage has had time to reach its maximum value for a given current then we say we are working in the time saturated region" and Equations V and VIII will apply;

General procedure for solution of ooerooltage problems The'generalprocedure to be followed in'the solution of any interrupted direct current overvoltage problem is as follows.

(a) Solve the corresponding stationary current case wherein current flows through the electrolyte and the metal objects'therein in accordance with Laplaces equationonly.

-(b) If we mayassume that the pulse duration time is so small that we are working in the time unsaturated region then we can use Equation XV to obtain the values of the overvoltageson' the cathodic and anodic faces of the metal particle.

(c) If we are working in the time saturate region then Equation VIII applies.

In either case, (b or 0 above) once we have the distribution of the overvoltage over the metallic body we can, assuming it to be caused by a double layer of charge, obtain the double layer moment or strength T from the relation ((1) the inducing current is removed the overvoltage decays. iGQOrding to Equation XII or XIII and since experimentally this is found where A is taken at the point of observation P. Therefore, the potential at P due to all the double layer distributions is where s includes all surfaces which have double layers.

(XVI) fda (XVII) Approximation for small current flow From Equation VIII we have, for the time saturated case,

where 0' is the conductivity of the electrolyte and E is the normal electric field into the metallic surface.

In the time unsaturated region we have i T V stE f iE=azaiE=AE If t is a constant pulse duration then A is a positive constant as well.

In the time saturated region we have, for small current densities e w' e o Z7: 1 7 in 41! bl; BB where B is a positive constant,

' is! 27r F'K It is worthy of note that E is the normal field into the metal and 7' is positive if there is a layer of positivecharge farthest from the metal and a layer of negative charge nearest the metal.

Application of method to geophysical prospecting Metallic sulphides are generally good conductors and when in contact with ground water and a current is caused to flow through the region overvoltage and double layer charge effects will be set up. The best way of detecting the overvoltage is through interruption of the inducing gelg and measurement of the'transient decay In order to predict what overvoltage effects are to be expected from a given volume of rock containing dlsseminatedsulphides :we make certain slmplifylng assumptions. Since it would be im-- lli possible to solve the current "distribution" problem for a general form ofparticle we shall assume that the particles under consideration are spherical in form. Secondly, these particles are assumed to be uniformly disseminated throughout the mass of rock. Lastly, not the whole surface of each metallic particle will be, in contact with the electrolyte. For example, ifr'the rock have 1 porosity then only Case of a spherical particle in uniform current flow , Refer to Figure 7.

Let the particle have a radius, a, and conductivity 0'1 and be imbedded in an electrolyte of conductivity 0'0. The potential function in the region exterior to the sphereis where E is the uniform field in which the sphere is placed.

The normal component of the electric field over the sphere is, then, r

( r-a c080 0+ l which, we must remember, is the outward field component.

The inward field component is given for (XIX) 30 E,-- E cos 0 (XX) and the outward field component for 30'1 E E cos B (XXI) In the time unsaturated region we have for the double layer strength 7 3171 -r-- 4m 2U0+GIE cos 0- m cos 0 V (XXII) where m i 301 V 4:7! 2170+ 0';

which holds for o o vr.

In the time saturated region i 5. Z 200+ 4w E cos 0- m coed N (XXIII) where 301 G l 2cr +a 41:- bR

12 Suppose the point at which-we are measuring' the potential isa distance r fromithercentcr of the particle and inclined at an angle 00 to the direction of the field E. By applying the Formula XVII, above, integrating over the surface of the sphere and evaluating to the first-power of '(i. e. to the order of) where 8 is the surface area-of the particle.

Thus, each spherical particle gives rise to a potential disturbance which is equivalent .to that of-a dipole of strength. I

r 1. xxx-i Type curvesand interpretation A practical method of interpretation may bebased on type curves, that is, theoretical curves representing the effects to be expected from specified geometrical distributions of metallic particles. Of particularinterest because of its greatadaptability is the case of a tabular body in a uniform field. This is supposed to have infinite strike lengthin the Y direction. I

Consider the case of metallic sulphide 563,139 tered in a tabular dipping body as shown in Figure 8. A uniform field E is considered acting parallel to the X axis and perpendicular to the strike of the body. The field at any point P ariliifig from the double, layer. charging effectsv W1 e r r em dcos d log 3 4..

by the solid curve of Figure ,8.-

In the actual practice'of our invention we ploy an array of three (3) pick-up electrodes P1, P2 and P3, the electrodes being uniformly spaced along the ground surface; the overvoltage The variation of Eo.v-. across the body is shown 3& 1,90%

2 computed with respect to the body of Figure ,8 is shown by the upper dotted curve of Figure 8.

,General method for detecting double layer effects Figure 9 is a simplified diagram to illustrate our method of geophysical exploration. An electrical field is established across an expanse of earth by the charging electrodes C, C1 having the indicated polarity when connected to a suitable source of D. C. voltage. As explained hereinabove, the charging field gives rise to the double layer charge effects at the surface of a metallic particle or body. The external charging field is thencut off and the double layer charge effect will dissipate itself in a matter of tenths of a second. The decay of the double layer charge is detected by an arrangement of pick-up electrodes P1, P2, P3; P1 being positive with respect to P2, and P2 being positive with respect to P3. When the outer pick-up electrodes are connected together by high ohmage resistors R1 and R2 of equal value, the resultant voltage appearing across the midpoint of the resistors and electrode P: will be p ii i v 2 l v As the resultant voltage depends upon the relative location of the pick-up electrodes with respect to the sub-surface metallic body, a resultant voltage of zero is indicative of one of three. possible conditions:

1. Absence of any metallic mass within the region defined by the downward projection of the pick-upelectrodes; or

2. A substantially uniform mass extending beyond the limits of the outer electrodes P1 and P3; or

3. The center electrode P2 lies over the center of a metallic mass which mass does not extend beyond the limits of the outer electrodes P1, P3.

The third condition, above, is of primary interest as will become more apparent hereinbelow.

Figure 10 illustrates graphically the relative relation between the charging current sent through the ground and the potentials appearing across the pick-up electrodes in the arrangement shown in Figure 9. Curve A shows the charging current flowing through the ground from the time to to time ti .afterwhich the charging current is cut oil until the next pulse is applied some time later.

Curve B shows the potential V1 appearing across the pick-up electrodes P1, P2 which, it will be noted, lie beyond the range of the metallic body (see Figure 9). The potential V1 results from the 1R drop across the earth between electrodes P1, P2 and, therefore, the curve B follows the curve A exactly. However, the potential V2 appearing across the pick-up electrodes P2, P3 (curve C) includes, in addition to the IR drop component which is equal to V1 (curve B), a residual component resulting from the double layer charge effects at the surface of the metallic mass lying between said electrodes P2, P3. As the double layer charge effects dissipate in a matter of, tenths of a second the curve C includes a decaying potential from the time ti (when the charging current has been cut off) to time is, it being again pointed out that the decaying potential duev to the double layer effect is in thesame direction as the charging current. If we now subtract Y1 from V: the resultant potential will be indcated by the curve D. It .will be noted that by subtracting, or bucking, the potentials V1 and V2 we eliminate all undesired voltage components as such components will, in most cases, be equal and of like direction between the equally spaced pickup electrodesPi, P2 and P2, P3.

' The peaks h'appearing on the resultant potential curve D represent the potential arising solely by reason of the double layer charge effects at the surfaces of the sub-surface metallic mass or particles. If now the pick-up electrodes be moved to the right in the Figure 9 arrangement the resultant peaks It will become somewhat smaller in height by reason of the fact that the pick-up electrodes P1, P2 approach the region of metallic body and the potential appearing across these electrodes will include a component of the double layer charge. If, then, we plot the resultant potential VZ-Vl (peaks h in curve D) against equal and known displacements of the center pick-up electrode P2 we obtain a curve as illustrated in Figure 11. The point at which this curve crosses the zero axis indicates the precise location of the center of the metallic mass. The importance, of such information in geophysical exploration is obvious.

Referring again to curve D, Figure 10, the height of the peak h and the character of the decay curve are indicative of the nature and composition of the sub-surface metallic particles. The use of type curves, as explained hereinabove, enables us to compute the expected double layer chargeefiects for various metallic particles lying at various distances below the surface of the ground. By correlating such data with experimental results obtained in the laboratory under known and controlled conditions we can adjust our apparatus such that the curves (curve D, Figure 10) obtained in field tests can be interpreted in terms of specific geological informaticn.

Specific apparatus We shall now describe in detail apparatus adapted to the practice of our method of geotion of thte pulser multivibrator is adjustable such that the charging current can be made to flow through the earth at regular intervals of time varying from .05 to 2.0 seconds and occuring at a rate which allows more time ofi than on; the proportion of oif" and on time being also adjustable as desired.

Pick-up electrodes P1, P2 and P3 are equally spaced along the ground within the limits of the charging electrodes and are connected to the D. C. amplifier through the equal and highohmage resistors R1 and R2. The ohmic value of resistors R1, and It; is high compared to the resistance of to theDLG, amplifier-"white iYFlfYi i Also;thefpick up"elctrods' prefefably areof the non-polarizing."si1ver"chl6ride or deeper sulfate typeeonrmoniyhsed in thear't'of geepnysiearexploration. A recorder reconnected ts-the output side of'tlieampl'ifier'. I Y

errengement 'eem fiomy identified as a clipper circuit is eemi eetee'tethe'bf oempiifier and "the pick-up electrodes. Its purpose is "to:

block the D. c; "amplifier so tnat no "indications will ja peer on the recerde eurih the t-p'er idd Whenthe'chargingfcurzfit is netting through the ea zth qrimvevet; chargin current is cutoff j the clipper "circuit instantly permitsithe" ampu'fi rto-open up wherebythe'potentiai-uinerg identified infthe block'diagram of Figure 1 2 now follows. I V

FiguredS is 'awiring diagram of 'the power supply. 'I'hefprimary winding of a high voltage 'tra'ngform'er this connected to a variable autotransrermerm thereby providing an arrangement wherebythe'voltage on the secondary side of thehighvoltagetransforiner can be varied from uto; 8000 volts. Rectifier tubes 32 and 33, which may be type 872, areconnectedasa' conventional full waverectifier including achoke input filter ohiprising the inductance and the condensates. The filaments of the tubes are enefg i'zed by a separate-step-down transformer 36. 'A- ble'e'der resistor s? connected across-the output o'f' tl-ie' rectifier circuit-may beemployed to 'inipr oire 'the regulatiomof the circuit as a whole. Irisei te'd in the rectified D; C; output circuit is ahig' h 'voltage, vacuum relay- 33" having aseven cooperating;

i rs ster tz i" "he d toetne 2 def A ube fiil and th other fi ee end to the grid} 'OFthe-tube5]' whiie -the" j6in d efils of said resistor 52 are connected to the ad'j-ustable arm of the resistor 55- connected between ground and the two plate dropping resistors 56 and 51. The bias of the multivibrator is set at a positive value 'by'tlie resister-se-to assure-stab oper n;

iiie squere'wevepeises rem t-he platefif be 5% are differentiated by the small condenser 58 and the grid rsistorsb'fi, tii'an'd thenega'tive peaks of the difierentiatedepulses are? byepa'ssed by' the" diode-'vacuumatube 'fil 1 The'yishapeseof the electrical:.pulsesrpassing through-this portionfiof th'e' oirouit are-shown th'e-areax-within the dotted lines on theupper partof Figure 1 4 -=Erom this it "will be noted only short positive .ipu-l'ses areapplied tothe grid fi? ofthe' 'vacunmtube-ifiii, the" "latter; together witha similar typQ 'SSN'? tube BA -"being arranged as a one shot multivibraton The posi-tive pulseapp'earing-on thetgrid {ii-lowers the voltage on the cooperating plate of-the tube 23 -and,=therefore, "results 4 in a; decreased voltage on the grid- GS of the tube 68 due ;tothe coupling condenser-5t. This deorease in -the voltage cuts ofiqthe current flow in the plateof the tube 64 and lowers the volta-ge drop-across the cathode resistor 6-! thereby-increasing the relative positive voltage on ther grid 5279i the tube 6-3 until the tube ti is entirelyoutoif. fl Tube- 6 remains cut off until the condenser 66 discharges throughathei adjustable resistor 68, at which time the circuit returns to the initial con'dition ready for the next short timing pulse to be applied to the grid 62. 'Each positive;pulse appearing 'on theegrfd 62 results, th'ereioiein a voltagetpulse on thezplat'e' of "tube 66 the length of whichcan be varied by the adjustable resistor 158, and the. frequency of which depends upon the freq'uency of osoillator section-of thecircuit. The output of theiplate-of contacts?!) and-t0; When the-contactse' ,Afi are" closed the ETC. charging current-flows through the charging electrodes. Operation of therelay contacts is controlled by'the' current fiowingin thefelay operating coil 41 which is connected in'tlie output side of a pulsen-multivibratornow talk-described. A voltmeter 42 and anammeter 43are inserted' in the-circuit to the charging.

electrodesto indicate the value of the voltage and'c'urifent durihgfthe periods when-pharging; current isffl'owihgthrough the earth. v 7

The wiring'diag ram ci the-pulser-multivibrator isshow'n in Figure I4. "The vacuum tubes 50-andplatesi arrd i g'ritis or -the' tubes 5o; one 1 free coupling condenser 10. Inasmuch astheop'er atin g -co'il of the highvoltage iz'acuum:relay. (See figure'lw' isconnected in-tha output-sidefof the tube 69 (as indicated by the legends) the vacuum relay wiil elose-and'openthe"cir"cuit to'the charging' electrodes 0, G1 (Figure 13) in exact oorre' spondence with the -outputi pulses of? the; pulser-' multivibra'tor. o I

"Figure--15 is-a wiring diagram of thetDs'C; balanced-amplifier. The-amplifieris'ofthe stable, direct coupled type the again of which-'-an-.-'-be varied: between :Zero' and l-,-000' 'without'afiecting the indicatiomofi a direct inking type of recorder connected-totheoutput circuit. This isracc'di'n; plislied by wse'ries ofi balanced biidge tubepircuits"; The pick up electrode arran'g'ehrent"described withreferencetd' Figure 9 is shown in the upperleft -corne'r of Figure 15; the "electrodes being. iden-t'ified-asPigPz and P3. Thep'ot'entialsiarising by virtueof "the-double layer'i charge 'eifeot'sare. picked up by the 'p'ick up electrodes --Pi,' P'2,J"Pa" an'd the potential V1 between theel'ectrod'e's F1. and P2- is subtracted from, or bu'cke'd *ag'ainsti! thegpotential Vzbetween the electro des Fiend-Psby' th'e' illustrated -circuit i'ncluding'the 'edualre sisters: Rl-Eilld R2. Thus'; at-potential- 'equal teen timest'o i .l

at pe'a t. )ti

1 7: voltage source and the voltage output of this network is,- in practice; adjusted. to equal .andsbuck out the normal earth and electrode voltage occurring 1 across the input terminals of the.

amplifier before the chargingcurrent is applied 5,

through the ground. Thus, these undesiredvoltages are-balanced or oancelledout of the circuit. As theam-plifier. has a directive sense with re-" spect to inpu-t voltage, the-amplifier outputwill represent the. potentialsar-ising=by virtue of the 10 double layer charge effects at the-surfaces ofsub-surface metallic bodies.

A switch 85 is composed-of a movable blade adapted to establish contact with :any onei of three stationary contacts. As shown in the draw-:

ing the switch is closed in .the'position marked L Indicate in. which positionthe circuit-isready for use. Whenthe switch is inthe position marked ..Calibrate the amplifierrinput is -.con-.

nectedto a'standard voltage source 86,.havingv a steady potential of 1.5 volts, connected .across a...voltage.divider 81 which is so adjusted and arranged that .01 voltincrementszcan be ob tained therefrom. When the switch .85 is set 'to the position marked .Short the amplifier is short circuited to ground under which condition no .voltage should be evident at the-output of the amplifier.

The resistor 88 is an input voltage divider that allows the overall voltage gain of-fthe amplifier.

to be adjusted asdesired.

The two tubes .89.and 90 (type 6K4) forma stablebridge circuit of nogain and low internal impedance to the ;grids-9I and .BZ-ofthefirstamplifying bridgecircult composed of the twoJi-fi sectionsofthetube 93. The resistors. 94 and 95 permitadjustment of the voltage levels of the grids. of the tubes. 93 and Strespectively for zero output on .the recorder. when the switch- 85 is set to the position marked Short. other armsof the input bridge comprise the resistors 96 and'91 which are connectedtoa- 19 L-voltbias,' as. shown A battery 98sup-. plies the plate voltage for thesinputbridges.

The plate resistor 99 and the left side of thetube93iormonearm of an amplifyingbridga with theplate resistor I00 and the'right side oftube 93as theother-arm. Resistor IUI provides a bias of .1.5 volts for the tube 93 and this resistor is also adjustable tosettheD. C. voltage levels between the plates of the tube toabout 22 volts. The gainof this bridge can be reduced .to zero without causing a voltage differencebetween the platesby increasing the-voltage drop across .the-

resistor- IOI .to 6 volts or-moreby theaddition of acur-rent from the-clippercircuit throughthe 1 connection to the terminal marked @Q or by all'owingcurrentto-flow through the resistor I92 by closingthe handtoperated switch I02A.

A signalapplied to-thegrid of tube'89 is am:-

plifiedand appears .betweenthe plates of tube 93. This signal isthen applied to the grids of the second amplifying, bridge composed of the tube 03 and-its supporting components: Resistor. I04 is a .dropping resistor adjustedto a value 55 to? obtain the proper voltage level across the platesofthe tube I03 and the resistor I05 :is'a balancing resistor used to adjust the output at. the recorder to zero when the-gain "isreduced to zero. The resistor I06 and the left side of the 7 tube I03 composeone arm' of an amplifying bridgeandthe resistor I01 and the'right side oithetube I03 :compose the other arm. A biasingyoltagedropfor the tube-I03 isprovided by the cathode resistor. I08 and the resistor .109.

of each tube.

lowers the gainand permits the amplifier to be calibrated against the. standard voltage drops chosen from across the calibrating resistor81;

The leftside of tube I I0- and resistor III together with the right sideof said tube I llland the resistor IIZ'fOrm a bridge of unity; gain and low internal impedance to drive the gridsof'a power bridge that energizes-the recorder. This power bridge is composed of thetubesl I3 and II 4 and the resistance arms II5 and H6,- The output voltage of the power bridge appears between the points and and the value of this voltage-is indicated-by ahighspeedrecorder of the direct.ink-ing-.type andhaving. a-sine wave frequency response up to 100 cycles per-second.

Figure 16 is a wiring. diagram of the clipper circuit which providesthe current to reduce the amplifier gain to zero during the time interval when .the charging-current is applied to theground I through the charging electrodes C, Ci-

Thecircuit comprises a --very low frequency. capacity. coupled amplifier of "conventional design which obtains apositive (see Figures 12 and :13).

signal from the pick-up electrodes Pl, P2 as shown in Figure 15. This signal is amplified byth'e tubes of the clipper circuit and results in a corresponding positive current pulse from the tube I20 (Figure 16) which current pulse is used to block the"amplifier.gain" during;the: period in" which the 'chargingi current flows through the earth, asexplained hereinabove" with reference to Figure 15. The tube I20 is biased well beyond" the cut-off" point and: the'amplifica'tion gain of the clipper circuit is'adjusted sothat a voltage input in the order of 2 volts (V1 of Figure 9) is required to actuate the clipper circuit. Inasmuch as the transient potentials arising by virtue of the double layer charge efiects have a maximum theoreticalivalueyof 1.3 volts, theclipper circuit is not afiectediby: such. potentials; Thus, when the charging current is flowing through the ground, a voltage of fsomething over 2-volts ap?- pears i across pick-up," electrode P1, P2 (IR drop 7 through ground) causing current to flow through the-cathode'oftube-120 (Figure 16)- and resistor IOI (Figure 15) thereby reducing the gain of theamplifier to zero; Theresistance-capacitance coupling. network between the stages of the clipper :circuitmust have a time constant comparable to theperiod of the-lowest frequencypulse usedforscharging theground. While the I pulses on .thegrids of the clipper circuit tubes flows through the cathode'of tube I20-(Figure- 16) and the amplifier gain'instantlyis returned to normal.

General .fielct operation 1 The above described apparatus, together with a motor-generator set,- may. becarried in a truck for mobile field operations. The charging-stakes and the pick-up: electrodes areconnectedto the apparatus by flexible cab1es-and are insertedinto the ground at desired positionsalong a profile 1 line of observation points; Self potentials; if any, exist'ing.at thepick-Hp electrodes ambalanced out by means of the variable voltage source described with reference to Figure 15. A

C, C1(Figu're 12). During the period when the D. C. charging current is flowing through the earth the clipper circuit (Figure 16) blocks the amplifier such that any potentials existing across the pick-up electrodes during these periods do not appear on the chart of the recorder. However, the instant the flow of D. C. charging current is out 01f the clipper circuit becomes ineffective and the amplifier (Figure 15) instantly responds to the voltage i -E-fi'.

arising by virtue of thedouble layer' charge effects developed at the sur'fac'es of the sub-surface particles. The magnitude of this voltage may be measured by means of a'ballistic' galvanometer; oscillographor direct inking pen type recorder.

The values of are plotted against positions of the center pickup electrode P2 (Fig. 11) and the magnitude, sign and shape of the obtained curve permit deductions to be made as to the possible width, depth, dip and metallic percentage of the causative mineral' or metallic concentration. Also, the rate of decay of the.

an indication of the specific metallic mineral present. V I

The herein described method of geophysical exploration is of equal validity when the pick-up electrodes P1; Pz,-and P3 are not confined to a line joining the charging stakes C, C1 or to an area within the limits of the charging stakes. In other words, the pick-up electrodes may be arranged in an angular array and such array can be inserted into the earth outside, or to one side, of the direct path of the charging current between'the charging stakes. In such cases, the voltagesv V1 and V2 (due to the'IR drop through the earth) willv not, in general be of equal mag-' nitude and the resultant curve 'z-Vr (curve D, Figure thus obtained requires the employment or a correction factor for proper geologicalinterpretations.

Further, while the method and apparatus have beendescribed with respect to an arrangement wherein the charging stakes and the pick-up ofthe invention as set forth in the claims.

We claim:

presence of sulphides in a medium permeated by an electrolyte said'method comprising: inserting a pair of charging stakes into the earth at spaced points; inserting an array of three pick-up electrodes'into the earth; applying a unidirectionalcharging current into the earth throughthe charging electrodes; interrupting said current within a time period of 30 seconds; and measuring the resultant transient potential appearing across, pairs of said pick-up electrodes, said measurement being restricted to the transient potential having the same directive sense as the charging current and which decays to one half peak value within 0.5 second.

2. The invention as recited in claim 1, wherein the pickup electrodes are arranged in line with each other and the charging stakes, and the pairs of pick-up electrodes are connected in elec-- trical opposition.

3. The geophysical method of detecting the presence of sub-soil sulphide particles in scatthat of the charging field and decaying to one half peak ,value within 0.5 second.

4. The geophysical method of determining the center of a mass of sub-soil metallic or mineral particles in a medium permeated by an electrolyte said method comprising: the insertion of a first, second and third pick-up electrode into the earth in the vicinity of the said mass, said first and third electrodes being equally spaced from the second electrode and all electrodes being in line; connecting the first and second electrode in electrical opposition to thesecondand third nately repeating the procedure of interrupting the current flow, measuring the resultant transient and moving the pick-up electrodes until the indication across the oppositely connected pickup electrodes is zero, under which condition the second pick-up electrode will be located directly opposite the center of the sub-soil mass.

5. Apparatus for geophysical exploration comprising a source of D. C. current, means for im. pressing a current flow from said source through the earth inthe form of pulses, detecting means for detecting potentials existing within the earths surface, and automatic means effective to prevent operationof the detecting means during the periods when the potentials exceed a predetermined inagnitude.

6. Apparatus for geophysical exploration comprising asource of D. C. current connected to a pair of charging stakes inserted into the earth,

a relay having an operating coil and provided with a set or" contacts connected between the source and one of the charging stakes, an electronic pulsing circuit controlling the energization of the relay operating coil, a set of pick-up electrodes inserted in the earth, detecting means connected to said pick-up electrodes to indicate transient potentials across said electrodes, and electronic means connected to said pick-up electrodes and to said detecting means said electronic means being efiective to prevent operation of said detecting means when the voltage across the pick-up electrode exceeds a predetermined magnitude.

7. The invention as recited in claim 6, wherein the detecting means comprises a direct coupled balanced electronic amplifier having a recorder connected in the output circuit.

8. The invention as recited in claim 7, wherein the pick-up electrodes comprise three in numher, a resistance connected across two of the pickup electrodes, and the amplifier is connected to the third pick-up electrode and the center point of said resistance.

9. Geophysical exploration apparatus for detecting the sub-surface presence of scattered or concentrated metallic or mineral particles in a medium permeated by an electrolyte said apparatus comprising: a source of D. C. current connected to a set of charging stakes inserted at spaced points in the earth; means effective to cause current from said source to flow through the earth between said stakes in pulses having a predetermined length and frequency, thereby establishing double layer charge effects at the surfaces of said particles; an array of three pick-up electrodes inserted into the earth; a pair of resistors connected across the outer pick-up electrodes; a direct coupled electronic balanced amplifier connected between the mid-point of said resistors and the center pick-up electrode; means for indicating the output of said amplifier; an electronic clipper circuit having its input connected directly to the center pick-up electrode and one of the outer pick-up electrodes, and its output applied to the cathode resistor of the first balanced voltage amplification stage of the amplifier; the recited combination resulting in the clipper circuit reducing the gain of the amplifier to zero during periods when the D. C. current is flowing through the earth.

10. The invention as recited in claim 9, wherein the means effective to cause current flow from the source through the ground comprises an electronic pulser-multivibrator; and including a power relay having its operating coil connected to the output of said pulser-multivibrator and having a set of contacts connected between the source of D. C. current and one of the charging stakes.

11. The invention as recited in claim 10, and including means effective to adjust the output current of the pulser-multivibrator to pulses of predetermined length and frequency.

12. Geophysical exploration apparatus for detecting the presence of sub-surface metallic and mineral particles in a medium permeated by an electrolyte said apparatus comprising: a source of D. C. current; a set of charging stakes inserted into the earth at spaced points said stakes being connected to the source of D. C. current; a relay having a set of contacts connected between the source of current and one of said stakes; a combination electronic oscillator and multivibrator; circuit elements connecting the output circuit of the multivibrator to the operating coil of said relay; adjustable means for altering the frequency of said oscillator; means for adjusting the lentgh of the current pulse in the output plate circuit of the multivibrator; an array of three pick-up electrodes inserted into the earth; a resistive component connected between the outer two pick-up electrodes; a direct coupled balanced amplifier having its input circuit connected across the center pick-up electrode and a point intermediate the ends of said resistive component; a source of fixed voltage; circuit elements for connecting the said source of fixed voltage to the input of the amplifier for calibrating purposes; means for applying an independent voltage of predetermined value to the input of said amplifier during periods when said input is connected to the pick-up electrodes; means connected in the amplifier output circuit for indicating the output of said amplifier; an electronic clipper circuit; circuit elements directly connecting the input of said clipper circuit between the center pick-up electrode and one of the outer electrodes; and means applying the output of said clipper circuit to the cathode resistor of the first balanced voltage amplification stage of the amplifier.

13. Apparatus for measuring the difference between two transient voltages each having a maximum predetermined value said apparatus comprising: a D. C. direct coupled balanced amplifier having an input and an output circuit; means connected in the amplifier output circuit to indicate the output of said amplifier; a first contact electrode; a second contact electrode; a third contact electrode; a resistive component connected across the first and third contact electrodes; circuit elements connecting the amplifier input circuit to the second contact electrode and a point intermediate the ends of said resistive component; an electronic clipper circuit arranged to produce an output current when the voltage applied to the input exceeds the maximum predetermined value of the transient voltages to be measured; circuit elements connecting the input of the clipper circuit across the first and second contact electrodes; and circuit elements connecting the output of the clipper circuit to the oathode resistor of the first voltage amplification stage of the said amplifier.

ARTHUR A. BRANT. EVERETT A. GILBERT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,153,636 Matsubara Apr. 11, 1939 2,190,321 Potapenko Feb. 13, 1940 2,190,323 Potapenko et a1. Feb. 13, 1940 2,190,324 Peterson Feb. 13, 1940 OTHER REFERENCES Geophysical Exploration, Heiland, pages 28- 30 and 744-754, published 1940 by Prentice Hall, Inc., N. Y. C. 

1. THE GEOPHYSICAL METHOD OF DETECTING PRESENCE OF SULPHIDES IN A MEDIUM PERMEATED BY AN ELECTROLYTE SAID METHOD COMPRISING: INSERTING A PAIR OF CHARGING STAKES INTO THE EARTH AT SPACED POINTS; INSERTING AN ARRAY OF THREE PICK-UP ELECTRODES INTO THE EARTH; APPLYING A UNIDIRECTIONAL CHARGING CURRENT INTO THE EARTH THROUGH THE CHARGING ELECTRODES; INTERRUPTING SAID CURRENT WITHIN A TIME PERIOD OF 30 SECONDS; AND MEASURING THE RESULTANT TRANSIENT POTENTIAL APPEARING ACROSS PAIRS OF SAID PICK-UP ELECTRODES, SAID MEASUREMENT BEING RESTRICTED TO THE TRANSIENT POTENTIAL HAVING THE SAME DIRECTIVE SENSE AS THE CHARGING CURRENT AND WHICH DECAYS TO ONE HALF PEAK VALUE WITHIN 0.5 SECOND. 